1. Field of the Invention
The present invention relates to the field of solid state telephone subscriber line interface circuits (SLIC's) and more particularly to circuitry for isolating and protecting such SLIC circuits from failure due to lightning induced ground overshoots. In accordance with the present invention, a circuit configuration is described wherein isolation through high ohmic value resistors is obtained which substantially reduces the susceptibility of the components of subscriber line interface circuits to damage from lightning.
2. Description of the Prior Art
Subscriber line interface circuits are well known in the prior art and examples of such SLIC circuits which are well known in the prior art are described, for example, in U.S. Pat. No. 4,161,633 of R. Treiber entitled Subcriber Line/Trunk Circuit; and in U.S. Pat. No. 4,254,305 of R. Treiber entitled Current Limited Subscriber Line Feed Circuit. Telephone equipment has in the past used transformer based circuits to interface to the telephone subscriber line. With the advent of improved cost and performance parameters of integrated circuit technology, solid state subscriber line interface circuits have been designed and are in use in the telecommunications industry. An example of a high voltage switch and capacitive drive circuit manufactured on an IC which is used in a telephone subscriber line circuit is described in U.S. Pat. No. 4,170,740 to J. Pernyeszi, entitled High Voltage Switch and Capacitive Drive. The subscriber line interface circuits of the known prior art, such as aforementioned, are difficult to protect from lightning induced voltages on the telephone subscriber line since such SLIC circuits are directly coupled to the line. Typical subscriber line interface circuits of the prior art use circuitry such as is described in FIG. 1 of the present application for protection from lightning induced voltages. The output stage of such a known prior art SLIC consists of two operational amplifiers configured as differential transconductance amplifiers wherein each such transconductance amplifier outputs a current which is linearly dependent on its differential input voltage. Such a SLIC output configuration is described in detail in the literature of the known prior art, such as in the IEEE Journal of Solid State Cicuits, Vol. SC-16, No. 4, Aug. 1981, pp 261-265. Such a prior art SLIC is powered from ground to battery voltage (-50 V) and operates around a Quiescent point at half battery voltage (-25 V). Such a circuit of FIG. 1 of the known prior art has the deficiency that any overshoot of battery ground (BG) is forced onto the output of the SLIC. Forcing the output to a level far above the low voltage ground (LVG), which is the most positive voltage of the SLIC output stage, usually results in destruction of the circuit components. In contradistinction, the circuit of the present invention provides an improved subscriber line interface cirucit which is not susceptible to lightning induced ground overshoots.